With the technology development and the growing demands for mobile devices, the demand for rechargeable electrochemical devices as an energy source is dramatically increasing, and research for electrochemical devices meeting various demands is being intensively conducted. Lithium secondary batteries may be given as a typical example of an electrochemical device.
A lithium secondary battery has a structure in which an electrode assembly is formed using a cathode and an anode, each manufactured by coating an active material on a current collector, with a separator interposed between the cathode and the anode, and the electrode assembly is impregnated with a non-aqueous electrolyte solution including a lithium salt.
The cathode and the anode are formed by coating a cathode mix slurry and an anode mix slurry on each current collector, respectively, and as a cathode active material, lithium cobalt-based oxide, lithium manganese-based oxide, lithium nickel-based oxide, and lithium composite oxide have been used, and as an anode active material, a carbon-based material has been mainly used.
Recently, a cathode active material having a high discharge capacity while sufficiently exhibiting high capacity characteristics, such as lithium nickel-based oxide, is attracting more attention. However, when an electrode mix slurry including a high capacity cathode active material is mixed by a general method, there are drawbacks of crystal structure destruction based on voltage zones and degradation of the properties of an anode solid electrolyte interphase (SEI) layer by generated metal ions.
As a graphite-based material being widely used as an anode active material nearly reaches its theoretical maximum capacity of 372 mAh/g, the graphite-based material exposes its limit in adequately serving as a quickly changing next generation energy source. In the case where lithium metal is used as an anode active material, high capacity may be achieved due to its very high energy density, but there is a safety issue and a short cycle life problem caused by dendrite during repeated charging and discharging. Besides, attempts have been made to use carbon nanotubes as an anode active material, but low productivity, high costs, and low initial efficiency of 50% or less of carbon nanotubes have been noted. Meanwhile, as it is known that silicon, tin, or an alloy thereof is capable of irreversibly intercalating and deintercalating a large amount of lithium through a compound forming reaction with lithium, recently research is being intensively conducted to use these materials as an anode active material. For example, due to having a theoretical maximum capacity of about 4020 mAh/g even higher than a graphite-based material, silicon is a promising high capacity anode active material.
A binder polymer serves for binding between active materials and between an active material and a current collector, and has a significant influence on battery characteristics by impeding volume expansion during battery charging and discharging. However, when an excessive amount of binder polymers is used to reduce a volume change during charging and discharging, separation of an active material from a current collector may diminish but electrical resistance increases due to an electrical insulation property of a binder polymer and an amount of active materials relatively reduces, resulting in reduced capacity.
More specifically, polyvinylidene fluoride (PVdF) being widely used as a binder polymer reduces in adhesive strength, brings about a safety issue, and causes environmental pollution by use of an organic solvent when preparing a slurry. To solve these problems of PVdF, a method which performs aqueous polymerization on an aqueous binder polymer such as styrene-butadiene rubber (SBR) and mixes it with a thickening agent and the like is proposed. An aqueous binder polymer is eco-friendly and has an advantage of a reduced content of binder polymer and consequently an increased battery capacity, but has a shortcoming of low electrical conductivity due to inclusion of polymer, namely, an insulation material.
A conductive material is added to an electrode mix to improve conductivity of an electrode. However, because a conductive material is hydrophobic, wettability is low and susceptibility of conductive material particles to agglomeration is great, so uniform dispersion and mixing with an active material or a binder polymer is impossible. Also, because a conductive material is incapable of permeating a binder polymer, namely, an insulation material, there is a limitation in improving electrical conductivity.
Recently, to meet the growing demand for a high capacity/low resistance electrochemical device in an electrochemical device market, the need for a fine grain size of a conductive material is increasing. In keeping up with a fine grain size of a conductive material, a conductive material reduces in size to a nanometer level, and as a consequence, susceptibility of a conductive material to agglomeration further increases, which makes uniform dispersion with an electrode active material and a binder polymer more difficult.
In particular, because a porous active material usually exhibit rheological properties of shear thickening characteristics, an electrode mix slurry using a porous active material has trouble with uniform dispersion with a conductive material.